A theoretical treatment of space charge effects in beam‐type magnetron amplifiers and oscillators is given. It is assumed that the beam is relatively thin and that the magnetic field is large. The ``cyclotron waves'' are not treated. A space charge parameter appears in this theory of magnetron‐type traveling‐wave interaction in a manner which is analogous to the manner in whichQCappears in ordinary traveling‐wave interaction. A distinctive feature of the space charge waves in the magnetron case is that one increases along the beam and the other decreases along the beam. A simple physical explanation of this effect is given.This theory is then used to determine the starting conditions of anM‐type backward wave oscillator. It is found that when the tube is long in space charge wavelengths there is an appreciable reduction of starting current. When the space charge parameter approaches zero, the solutions found here reduce to the usual two‐wave solutions.